Modern day PABXs supply operating power to subscribers' lines at −48 volts, applied to tip and ring leads of the lines. The length of a subscriber's line or subscriber loop governs its resistance. Hence, the current drawn by a long subscriber loop is substantially less than that drawn by a short loop.
Prior art line circuits typically did not compensate for different loop lengths and usually provided a constant voltage battery feed to the tip and ring leads. A minimum loop current of approximately 20 mA is typically required to be provided to a subscriber's loop in order to operate a telephone set connected to the loop. For long loops, large voltages were required in order to generate the minimum 20 mA current. Conversely, considerable power was wasted for short subscriber loops as a result of driving the line with unnecessarily high voltages (eg. −48 volts, −96 volts, etc.).
The problem of prior art line circuit power regulation is exemplified by the well known transformer hybrid which includes a large magnetic core for passing D.C. feed current to the loop. The core is made large in order not to saturate in the presence of large D.C. feed currents. Prior art transformer hybrids were bulky and expensive, and thus did not conform to miniaturization requirements of modern day PABXs.
Commonly assigned U.S. Pat. No. 4,723,280 (Meier) sets forth a constant current line circuit which utilizes a differential amplifier for transmitting audio signals to and from a subscriber's loop as well as detecting the amount of feed current flowing in the loop. The differential amplifier compares the detected feed current to a threshold value for establishing the constant current, and generates a D.C. voltage signal which varies in amplitude in response to variations in the amount of feed current flowing in the loop, resulting from variations in loop resistance, etc. The D.C. signal is passed through an integrator circuit in order to remove audio signal components and is applied to a voltage controlled D.C. power supply for increasing or decreasing feed voltage applied to the loop in response to a decrease or increase respectively in the feed current flowing in the loop, thereby maintaining constant feed current. The D.C. signal generated by the differential amplifier is regulated by the integrator circuit to a quiescent value of approximately zero volts, such that audio signals received from the subscriber's loop can be applied directly to an unbalanced transmit lead connected to the PABX, without requiring separate D.C. blocking signal translation circuitry.
The prior art constant current feed driver suffers from two significant weaknesses which prevent it from being used to provide CLASS services (e.g. caller ID). The first problem is that the ring driver is driven to saturation in the on-hook state. This prevents the transmission of the caller ID information. The second weakness is that the circuit ceases to function if the constant current requirement cannot be satisfied (e.g. if the loop range exceeds the design criteria or the terminal device or devices require more current than the circuit can provide). Specifically, once the ring driver is driven to saturation, the zero volt quiescent output of the differential amplifier cannot be maintained, resulting in incorrect tip bias.